In X-ray scatterometry measurements, a beam of X-rays is incident on a sample, and the intensity distribution of the X-rays scattered from the array of features is measured by a suitable detector. Some measurement modalities operate in a reflective geometry, in which an X-ray beam is incident on one side of the sample and is reflected (by specular and/or diffuse reflection) from the same side of the sample. Such reflection generally takes place only with X-ray beams incident at grazing angles, i.e., low angles of incidence, generally along a beam axis that is within 4° of the surface, and often below 1° from the surface.
Because of the low angle of incidence, the incident X-ray beam extends over an elongated area of the surface, referred to as the beam “footprint,” and the scattering measurements have correspondingly low spatial resolution and require large measurement areas. To enhance the spatial resolution, some scatterometry systems use a knife edge, placed above the sample at the point at which the X-ray beam is incident on the surface. The knife edge is positioned so as to create a small gap between the blade and the surface of the sample through which the X-rays must pass in order reach the detector, thus reducing the effective length of the footprint. In the context of the present description and in the claims, the term “knife edge” refers to any type of straight edge (not necessarily very sharp) that is positioned near the surface of a sample in order to create a small gap between the knife edge and the surface and to block X-rays outside the gap.
An arrangement of this sort is described, for example, in U.S. Pat. No. 6,512,814, whose disclosure is incorporated herein by reference. This patent describes reflectometry apparatus, which includes a radiation source, adapted to irradiate a sample with radiation over a range of angles relative to a surface of the sample, and a detector assembly, positioned to receive the radiation specularly reflected from the sample over the range of angles and to generate a signal responsive thereto. A shutter is adjustably positionable to intercept the radiation. The shutter has a blocking position, in which it blocks the radiation in a lower portion of the range of angles, and a clear position, in which the radiation in the lower portion of the range reaches the array substantially without blockage. A dynamic knife edge is positioned over the surface. Preferably, the dynamic knife edge is operated in conjunction with the above-mentioned dynamic shutter. For measurements at low incidence angles, the knife edge is lowered very near to the surface, intercepting the incident X-ray beam and thus reducing the lateral dimension of the spot on the surface (i.e., the dimension in the direction along the surface that is roughly parallel to the beam axis).
U.S. Pat. No. 7,551,719, whose disclosure is incorporated herein by reference, describes another multifunction X-ray analysis system, which combines X-ray reflectometry (XRR) with small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) measurement. In one embodiment, shown in FIG. 5 of this patent, a knife edge is made of a cylindrical, X-ray absorbing material, such as a metal wire. This arrangement is said to permit the lower edge of the knife to be placed very close to the surface of the sample, on the order of 3 μm above the surface, without risk of damaging the sample. The wire can be aligned with the surface accurately and thus provides a small gap above the surface whose effective height is uniform over the entire angular range of interest, typically 0-4°.
U.S. Pat. No. 7,406,153, whose disclosure is incorporated herein by reference, describes apparatus for analysis of a sample that includes a radiation source, which is configured to direct a beam of radiation along a beam axis to impinge on a target area on a surface of the sample. A detector assembly is configured to sense the radiation scattered from the sample. A beam control assembly includes a beam blocker, which has a lower side adjoining the surface of the sample, and which contains front and rear slits perpendicular to the lower side that together define a beam plane that contains the beam axis and passes through the target area. In some embodiments, a beam limiter is positioned within the beam plain so as to block a portion of the plain. The beam limiter has a knife edge, which is transverse to the beam plane and typically protrudes below the lower side of the beam blocker.